1. Field of the Invention
The invention relates to an image reading apparatus, and particularly to an image reading apparatus adapted to read an image formed on a sheet while transporting the sheet.
2. Related Background Art
As an image reading apparatus for reading an image formed on a sheet, there is, for example, one of an original transporting type which is adapted to read an image formed on an original which is a sheet while transporting the original. As such image reading apparatuses of the original transporting type, there are known facsimile apparatuses, scanners, copying machines, etc.
Now, among these apparatuses, there is one of a type which reads the images of the front side and back side of an original at a time. FIG. 12 of the accompanying drawings shows an example of such an image reading apparatus according to the conventional art, and this image reading apparatus is provided with a transport path for U-turning an original P, i.e., a so-called U-turn path, in order to make the area occupied by the apparatus small.
When originals P are to be read, the originals P are first set with their front sides facing upwardly on an original feed tray 401 on the front face (the right side as viewed in FIG. 12) of the apparatus, and the uppermost one of the thus set originals P is separated by a separation portion 402, and is fed into image reading portions 404 and 405 by a pair of transport rollers 403.
Next, the images on the front side and back side of this original P are read by image sensors 404a and 405a, respectively, provided in the image reading portions 404 and 405. Platen rollers 404b and 405b are disposed in opposed relationship with these image sensors 404a and 405a, respectively, and the original P is brought into close contact with contact glass surfaces on the respective image sensors by these platen rollers 404b and 405b, whereby the reading of the images of the original can be done well.
The original P of which the images on the front side and back side have been read in the this manner is thereafter passed through a U-turn guide portion 407 by a pair of transport rollers 406, and is stacked with its front side facing downwardly on an original delivery tray 409 provided on the upper surface of the apparatus, by a pair of delivery rollers 408.
According to the image reading apparatus of such a construction, the area occupied by the apparatus becomes relatively small and moreover, the take-in and delivery of the original P are in the same direction and therefore, the setting and collection of the originals P can be effected easily.
On the other hand, FIG. 13 of the accompanying drawings shows another example of the image reading apparatus according to the conventional art, and this image reading apparatus is of a type in which originals P are set on the upper portion of the rear side (the right upper side as viewed in FIG. 13) of the apparatus with their front sides facing downward, and are transported substantially linearly.
When the originals P are to be read, the originals P are first set on an original feed tray 401 with their front sides facing downward, and the uppermost one of the thus set originals P is separated by a separation portion 402, and thereafter is fed into image reading portions 404 and 405 by a pair of transport rollers 403.
Next, the images on the front side and back side of this original P are read by image sensors 404a and 405a provided in the image reading portions 404 and 405, respectively. Thereafter, the original P of which the images on the front side and back side have been thus read is stacked on an original delivery tray 409 in the lower portion of the front side (the left lower side as viewed in FIG. 13) of the apparatus with its front side facing downward by a pair of delivery rollers 408.
According to the image reading apparatus of such a construction, the transport path of the original P is a so-called straight path approximate to a straight line and therefore, there is the advantage that relatively stable transport of the original P can be effected and moreover, the transport rollers and the transport guides can be made into a necessary minimum construction. Further, if the angle of the straight path is made great with respect to a horizontal plane and the angle of the original feed tray 401 is made upright and the original P is transported at an angle approximate to verticality, the area occupied (projected) by the entire apparatus can be made small.
However, in such an image reading apparatus according to the conventional art, for example, in the image reading apparatus as shown in FIG. 12, to make the entire apparatus compact, it is necessary to make the curvature of the U-turn guide portion small, but if the curvature is made small, not only it will become impossible to transport a thick original having stiffness, but also the accuracy of transport will be reduced and bad transport will become liable to occur.
That is, when an original is transported by the use of a U-turn path small in curvature, the entire apparatus can be made compact, but there is the disadvantage that as compared with the straight path, the range of application to the thickness of the original becomes narrow. There is also the disadvantage that transport rollers and a transport guide become necessary in the U-turn portion and the construction becomes complicated.
On the other hand, in the image reading apparatus as shown in FIG. 13, if the angle of the straight path is made great with respect to the horizontal plane and the original is transported at an angle approximate to verticality, the area occupied (projected) by the apparatus can be made small, but in this case, it is necessary to dispose the original delivery tray 409 at an angle approximate to verticality.
However, if the original delivery tray 409 is disposed at an angle approximate to verticality as described above, there is the disadvantage that the position of the original delivery tray 409 becomes high relative to the installation surface and the height dimension of the apparatus becomes great and the operability of the apparatus becomes bad. Accordingly, it is more advantageous in the compactness and operability of the apparatus for the original delivery tray 409 to be disposed at an angle as approximate to horizontal as possible.
However, if the original delivery tray 409 is disposed at an angle approximate to horizontal as described above, the transport direction of the original P transported at an angle approximate to verticality will come to change greatly short of the original delivery tray 409. If the original delivery angle changes greatly like this, there has been the disadvantage that it becomes impossible to deliver a thick original having stiffness or the delivered original becomes bent or delivery jam becomes liable to occur.
Also, in recent years, an image forming apparatus such as an image reading apparatus, a printer, a copying machine or a facsimile apparatus is provided with a sheet feeding apparatus for separating sheets one by one and feeding them to an image reading portion or an image forming portion. As such a sheet feeding apparatus, use is widely made of one adopting a retard separation type capable of separating sheets with high reliability from a low speed to a high speed, as a mechanism for separating the sheets.
As shown, for example, in FIGS. 14A, 14B and 14C of the accompanying drawings, the retard separation type is adapted to separate and feed sheets S one by one by a feed roller 411 rotated in a sheet feeding direction (the direction indicated by the arrow A in FIG. 14A), and a separation roller 412 (retard roller) urged against the feed roller 411 with predetermined pressure and rotated in a direction opposite to the sheet feeding direction, in other words, a direction for returning the sheets S.
A torque limiter for generating slide at a prescribed or greater load is provided in the driving force transmitting path of the separation roller 412. This torque limiter is usually adapted to work by a load comprising the frictional force between the feed roller 411 and the sheet S. Thereby, the separation roller 412 is adapted to follow the rotation of (be rotated with) the sheet S, and transport a sheet.
On the other hand, when a plurality of sheets S overlapping one another come into the nip between the rollers, a sheet S which is brought into contact with the feed roller 411 by the slide among the sheets S is transported in the feeding direction. A sheet which is in contact with the separation roller 412 side is adapted to be returned by the separation roller 412 being reversely rotated.
Further, when multiple sheets S superposed one upon another are set, the stack of the sheets dashes into the nip between the rollers and the double feed of multiple sheets occurs. In order to prevent it, there is also known an apparatus in which as shown in FIG. 14B, an inclined surface portion is formed on this side of the separation roller 412 or as shown in FIG. 14C, a wall portion is formed to thereby provide such a guide portion 420 that a stack of a predetermined number of or more sheets does not dash into the nip.
However, in the above-described examples of the conventional art, particularly in the case of a longitudinal type sheet feeding mechanism, a stack of multiple sheets superposed one upon another is directly placed in the roller nip portion of the transport and separation device and therefore, the above-described operation of the separation roller 412 is blocked to thereby cause double feed and bad feed. Also, even if the guide portion 420 shown in FIG. 14B or 14C is provided, sheets having a high coefficient of friction come into between the guide portion 420 and a transport guide more than necessary, with the aid of gravity of the sheets S, to thereby cause the double feed of multiple sheets or the non-feed by sheet jam.